A federal appellate court has upheld a judge's decision to fine Carnival Corp $20m for violating probation tied to environmental crimes.

The Eleventh Circuit Court of Appeals has denied a 17 June challenge by four people who alleged they were harmed by Arnold Donald-led Carnival's actions, including dumping garbage into the sea.

The appeal had been filed by Fotini Tsavousis Duncombe, co-founder of Bahamas environmental group reEarth, Responsible Cruising in Alaska president Theodore Thoma, and retired Alaskan fishermen Ronn Buschmann and Eric Forrer.

"To say that the denial of our rights as victims of Carnival Corp's crimes is disappointing would be putting it mildly," Duncombe said.

"We have been living with the impacts of Carnival’s environmental crimes for decades."

An email to Carnival seeking comment was not immediately returned.

The four "victims" on 31 May filed an emergency motion in Miami federal court to intervene in the court proceedings ahead of a 3 June hearing.

They asked the federal district court to recognise their right to participate in the case under the Crime Victims’ Rights Act.

Their comments recounted how Carnival’s pollution hurt their livelihoods and quality of life and asked US District Judge Patricia Seitz to consider this in weighing punishment.

Carnival has served two years of the five-year probation as part of a $40m settlement for illegally dumping oil into the ocean from its Princess Cruises ships for eight years and lying about it to US authorities.

While on probation, Carnival and its subsidiary cruise lines tried to avoid unfavourable findings by preparing ships in advance of court-ordered audits and falsifying records.

The New York-listed company also dumped plastic garbage into the ocean and illegally discharged grey water into Glacier Bay National Park in Alaska.

The company also has tried to lobby the US Coast Guard through a back channel to change the terms of the settlement, prosecutors allege.

The company has acknowledged these incidents, according to court filings.

In the appeal, the environmental groups and fishermen claimed they were denied their right to submit written testimony as participants in the 3 June hearing under the Crime Victims Rights Act (CVRA), according to court documents.

"These victims sought to exercise their procedural rights under the CVRA, but did not seek any restitution," the documents state.

The appellants also argued, among other things, that the government did not share the proposed $20m plea agreement until one hour before the hearing was held.

The cost of Carnivals' violations should be higher than just the cost of doing business, said Knoll Lowney, the appellants' attorney.

"We don’t want Carnival to continue its 'criminal as usual' business practices," he told TradeWinds.

Lowney said the case is "over for now" since the court has rejected the appeal.

The appellants plan to work with US-based environmental group Stand.earth to "hold Carnival accountable", said Lowney said.

"We'll be organising communities and using environmental and consumer protection laws to force Carnival to clean up its business,” Lowney said.

Stand.earth said it will continue to fight for the appellants' rights to a clean environment, despite the uphill battle they face against such a large company as Carnival.

"While the courts have again utterly failed these victims, we will continue to stand with them to fight for their right to be free from Carnival’s criminal pollution,” said Kendra Ulrich, senior shipping campaigner at Stand.earth.